Chirped fiber acousto-optic bandpass filter

ABSTRACT

A device whereby one or more bands of optical wavelengths may be selected for further transmission. All light within the optical bandwidth of operation is first coupled from the core mode of an optical fiber to a specific cladding mode by a chirped broadband cladding mode coupler. These cladding mode lightwaves then enter a narrow-band core mode coupler whereby selected optical bands of wavelengths are re-coupled back into the core of the optical fiber. The chirped broadband cladding mode coupler is isolated from the narrow-band core mode coupler by an acoustic absorber to limit the acoustic interaction between them.

BACKGROUND OF INVENTION

1. Field of the Invention

This invention relates generally to telecommunication systems andassemblies, and more particularly to a chirped fiber acousto-opticbandpass filter.

2. Description of the Related Art

An important function in the telecommunication industry is signalswitching. The switching can be performed either electronically oroptically. In past years, this switching was accomplished throughelectronic means. However, with the increasing demand for lower cost,higher switching speeds, lower power consumption, and lower opticallosses, optical switching is becoming more commonplace. There are twotypes of optical switches currently used, wavelength insensitive opticalswitches and wavelength sensitive optical switches. The wavelengthinsensitive optical switches are typically broadband fiber-to-fiberswitches used to redirect all the traffic from one optical fiber toanother. Because the switching process is either thermo, electro-optic,or mechanical, the switching speed is slow but satisfactory. Howeverthese switches do not satisfy the requirements for low cost, highreliability, and low optical insertion loss.

The wavelength sensitive optical switches are needed for wavelengthdivision multiplexed (WDM) signals because the wavelength separationbetween channels is small. A narrow optical band of traffic carried by aspecific wavelength of a multi-wavelength signal may need to beseparated from the rest of the traffic. A wavelength-sensitive opticalswitch can perform this function optically at considerable cost savings.Existing wavelength sensitive optical switches are usually bulky, havehigh power consumption, and high optical insertion losses. For instance,in a previous patent by this inventor entitled Tunable Optic FiberBandpass Filter Using Flexural Acoustic Waves, U.S. Pat. No. 6,151,427,an acousto-optic bandpass filter was described, however that inventionuses a core block that introduces significant optical insertion losses,added complexity, and is costly to manufacture. The present inventiondoes not require a core block component, thereby negating these problemsand simplifying the architecture. Other acousto-optic filters include“Acousto-optic Filter,” U.S. Pat. No. 6,233,379 by Kim et al, which ishereby incorporated by reference. The filter described performs thefunction of a band-stop filter and can select a limited number ofoptical bands (channels) simultaneously but does so at the cost ofincreased power consumption for each band selected to the limit of theacousto-optic generator. The current invention eliminates all theoptical bands (channels) simultaneously and can then select one or morechannels to pass through the filter, thus performing a bandpassoperation.

SUMMARY OF INVENTION

In consideration of the problems detailed above and the limitationsenumerated in the partial solutions thereto, an object of the presentinvention is to provide an improved chirped fiber acousto-optic bandpassfilter that does not require a core-block and uses less electricalpower.

Another object of the present invention is to provide a chirped fiberacousto-optic bandpass filter with multiple acoustic signals that haveindividual controllable strengths and frequencies.

Yet another object of the present invention is to provide a broadbandcladding mode coupler to efficiently couple all the optical traffic froma fiber core mode to a fiber cladding mode for later selection ofindividual optical channels.

Yet a further object of the present invention is to provide an all-fiberchirped acousto-optic bandpass filter that includes an optical fiberwith a core and a cladding where the strength or the magnitude of anoptical signal coupled from the cladding to the core is changed byvarying the amplitude of electrical sinusoidal frequency applied to anacoustic wave generator.

In order to attain the objectives described above, according to anaspect of the present invention, there is provided a chirped fiberacousto-optic bandpass filter whereby one or more bands of opticalwavelengths may be selected for further transmission. In this device,all light within the optical bandwidth of operation is first coupledfrom the core mode of a optical fiber to a specific cladding mode withina first acousto-optic interaction region, a chirped broadband claddingmode coupler, where a selected RF frequency of a flexure wave, inducedby a first acoustic wave amplifier, acting on a region of the opticalfiber that has been chirped, couples all light within the opticalbandwidth of operation from the core mode of the optical fiber to aspecific cladding mode. These cladding mode lightwaves then enter asecond acousto-optic interaction region, a narrow-band core modecoupler, where selected frequencies of flexure waves, induced by asecond acoustic wave amplifier, re-couple selected bands of wavelengthsback into the core mode. The second acousto-optic interaction region isisolated from the first acousto-optic interaction region by an acousticabsorber to limit acoustic interaction between the acousto-opticinteraction regions.

The aforementioned features, objects, and advantages of this method overthe prior art will become apparent to those skilled in the art from thefollowing detailed description and accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

My invention can best be understood when reading the followingspecification with reference to the accompanying drawings, which areincorporated in and form a part of the specification, illustratealternate embodiments of the present invention, and together with thedescription, serve to explain the principles of the invention. In thedrawings:

FIG. 1 is a schematic diagram showing one embodiment of the chirpedfiber acousto-optic bandpass filter that contains the chirped broadbandcladding mode coupler, the acoustic absorber, and the narrow-band coremode coupler.

FIG. 2 is a schematic diagram of another embodiment of the chirped fiberacousto-optic bandpass filter that contains the chirped broadbandcladding mode coupler and the narrow-band core mode coupler with atapered interaction region.

FIG. 3 is a schematic diagram of yet another embodiment of the chirpedfiber acousto-optic bandpass filter that contains the chirped broadbandcladding mode coupler with a tapered interaction region, the narrow-bandcore mode coupler, also with a tapered interaction region.

FIG. 4 is a cross-sectional view of the optical fiber.

FIG. 5 is a cross-sectional view of a high index acoustic absorberconfiguration.

FIG. 6 is a detailed view of the acoustic wave amplifier.

DETAILED DESCRIPTION

In one embodiment of a chirped fiber acousto-optic bandpass filter(herein bandpass filter) 30, the present invention, as shown in FIG. 1,one or more optical wavelength bands may be selected for furthertransmission. In this device, all lightwaves within the opticalbandwidth of operation are first coupled, in a chirped broadbandcladding mode coupler 34, from the core mode of a non-birefringentsingle-mode optical fiber (herein fiber or optical fiber) 32 to aspecific cladding mode using a fiber 32 whose diameter is altered in alinear (chirped) fashion within the first acousto-optic interactionregion 43. In the chirped broadband cladding mode coupler 34, a firstacoustic-wave generator 36 produces a mechanical vibration at a fixedfrequency provided by the first frequency source 52. The mechanicalvibration creates acoustic waves that travel from the base 56 to the tip40 of the first acoustic wave amplifier 38. The acoustic waves arecoupled to the optical fiber 32 at the tip 40 of the first acoustic waveamplifier 38. The acoustic waves traverse along the first acousto-opticinteraction region 43 of the optical fiber 32 as a traveling flexurewave significantly coupling the lightwaves from the core mode to acladding mode within the optical fiber 32. These cladding modelightwaves then enter a narrow-band core mode coupler 35 which has asecond acousto-optic interaction region 44 where selected flexure wavefrequencies, induced by a second acoustic wave amplifier 46, re-coupleselected narrow bands of optical wavelengths back into the core mode ofthe optical fiber 32. The second acousto-optic interaction region 44 ofthe narrow-band core mode coupler 35 is isolated from the firstacousto-optic interaction region 43 of the chirped broadband claddingmode coupler 34 by an acoustic absorber 48 to limit acousticinteraction. The core mode coupling of a narrow band of opticalwavelengths within the narrow-band core mode coupler 35 may be opticallytuned by varying the electrical sinusoidal frequency of the secondfrequency source 54 that is electrically connected to the secondacoustic wave generator 37, which in turn creates a mechanical vibrationfrequency. The acoustic wave generator 37 is attached to the base 57 ofthe second acoustic wave amplifier 46. The generated acoustic wavestraverse through the second acoustic wave amplifier 46 and are coupledto the fiber 32.

The coupling within the chirped broadband cladding mode coupler 34 isaccomplished by generating an electrical sinusoidal frequency using afirst frequency source 52 that is electrically attached to the firstacoustic wave generator 36. The electrical sinusoidal signal causes amechanical vibration of the first acoustic wave generator 36 thatlaunches acoustic waves from the base 56 to the tip 40 of the firstacoustic wave amplifier 38. The acoustic waves are then coupled to thefiber 32 and propagate as traveling flexure waves within the firstacousto-optic interaction region 43 and terminate at the acousticabsorber 48. The traveling flexure waves create a microbend structure inthe fiber 32 within the first interaction region 43. The microbendstructure induces an asymmetric refractive index change in the fiber 32,and thereby couples lightwaves from a symmetric core mode to anasymmetric. cladding mode. For efficient mode coupling, the period ofthe microbending, or the acoustic wavelength, should match the beatlength defined by the coupled modes. The beat length is defined by theoptical wavelength divided by the effective refractive index differencebetween the two modes. Because of the chirped diameter of the fiber 32within the first acousto-optic interaction region 43, the acousticwavelength and thus the optical wavelength band being coupled to thecladding will change as the lightwaves propagate through the firstacousto-optic interaction region 43, causing a broadband of opticallightwaves to be coupled from the core mode to the cladding mode.

At least one first acoustic wave generator 36 is coupled to the base 56of the first acoustic wave amplifier 38 which mechanically vibrates,typically at a frequency in the range of 1-40 MHz. The first acousticwave generator 36 is preferably deployed as a shear mode transducer. Thefirst acoustic wave generator 36 can be made at least partially of apiezoelectric material whose physical shape is changed in response to anapplied electric sinusoidal voltage induced by the first frequencysource 52. Suitable piezoelectric materials include but are not limitedto quartz, lithium niobate, zinc monoxide, and PZT, a composite of lead,zinconate, and titanate.

The narrow-band core mode coupler 35 is used to select individualoptical wavelength bands for transmission through the bandpass filter30. The second acoustic wave generator 37 may produce acoustic waveshaving various frequencies with individual controllable amplitudes tocouple cladding modes of a particular optical wavelength back into thecore 70 of the fiber 32. Each of the acoustic waves may provide acoupling between the cladding mode and the core mode within the secondacousto-optic interaction region 44. Thus, selected optical wavelengthsof the signal may be converted from the cladding mode to the core modeby choosing the appropriate electrical sinusoidal frequencies for thesignal applied to the acoustic wave generator 37.

As the lightwaves propagate as a cladding mode along the fiber 32 andpast the acoustic absorber 48, a desired band of optical wavelengths maybe converted back to the core mode provided the phase-matching conditionis satisfied according to: L_(b)=Λ_(a)=2π/(β_(co)(λ)−β_(cl)(λ)), whereΛ_(a) represents the acoustic wavelength of the traveling flexureacoustic waves, L_(b) represents the beat length, and β_(co)(λ) andβ_(cl)(λ) are optical wavelength dependent propagation constants of thecore mode and the cladding mode, respectively. When the acousticwavelength Λ_(a) is equal to the beat length L_(b) defined by the twomodes, the phase-matching condition is satisfied and conversion betweenmodes occurs. The remaining cladding mode lightwaves that are notconverted to the core mode get absorbed in the buffer material 74 of thefiber upon exiting the bandpass filter 30 or may be absorbed by thebonding agent 50 of the second acoustic wave amplifier 46. The desiredoptical wavelength band may be tuned by adjusting the electricalsinusoidal frequency of the second frequency source 54 and the couplingefficiency may be tuned by adjusting the amplitude of the electricalsinusoidal frequency applied to the second acoustic wave generator 37.

Accordingly, if the frequency of the electrical sinusoidal signalapplied to the second acoustic wave generator 37 varies, the acousticwavelength generated within the second acousto-optic interaction region44 also varies, which results in the passage of a different band oflightwaves by the bandpass filter 30. In addition, since the magnitudeof the optical signal that is transmitted is dependent of the amplitudeof the traveling flexure acoustic wave, the optical signal strength canbe adjusted by varying the amplitude of the electrical sinusoidal signalthat is applied to the second acoustic wave generator 37.

In another embodiment of the bandpass filter 30, as illustrated in FIG.2, the fiber 32 within the second acousto-optic interaction region 44 ofthe narrow-band core mode coupler 35 is tapered. This uniformly taperedoptical fiber 60 provides enhanced conversion efficiency and allows fora shorter second acousto-optic interaction region 44. As shown, theuniformly tapered optical fiber 60 has a section where the diameter ofthe fiber 32 is uniformly narrowed for some length. This uniformlytapered optical fiber 60 may be created by a variety of methodsincluding, but not limited to, a pulling process using a traveling torchas described in Acousto-optic Filter, U.S. Pat. No. 6,233,379, by Kim etal. The tapering process causes the core 70 of the optical fiber 32 tobe eliminated within the uniformly tapered optical fiber 60. The opticallightwaves within this uniformly tapered optical fiber 60 then propagateas cladding modes within the optical fiber 32. The propagation constantsof supported optical modes can be greatly changed by a diameter changeof the optical fiber 32 in the second acousto-optic interaction region44. Additionally, the internal stress distribution is modified by stressannealing induced by the flame. Because the diameter of the fiber 32within the second acousto-optic interaction region 44 will besignificantly reduced, higher electrical sinusoidal frequencies will beneeded to generate a shorter acoustic wavelength, Λ_(a). The shorteracoustic wavelength is necessary to convert the higher order claddingmode into the lower order cladding mode at the desired opticalwavelength. These lower order cladding mode lightwaves propagate intocore modes beyond the uniformly tapered optical fiber 60 as the fiber 32returns to its standard diameter. A more detailed discussion of thetheory and experimental results are included in a paper entitledNarrow-Band Acousto-Optic Tunable Filter Fabricated From A HighlyUniform Tapered Optical Fiber by Dimmick, et al. and is hereby includedas a reference. The resultant effect of using a uniformly taperedoptical fiber 60 within the second acousto-optic interaction region 44is an improved cladding mode to core mode conversion efficiency.

In yet another embodiment of the bandpass filter 30, the fiber 32 in thefirst acousto-optic interaction region 43 of the chirped broadbandcladding mode coupler 34 is tapered and chirped as illustrated in FIG.3. This tapering provides enhanced conversion efficiency and allows fora shorter length of the first acousto-optic interaction region 43. Inaddition, the tapering of the fiber 32 maybe extended into thenarrow-band core mode coupler 35 and the second acousto-opticinteraction region 44, providing the advantages described above.

The fiber 32 has a core 70 and a cladding 72 surrounding the core in aconcentric fashion as illustrated in FIG. 4. Surrounding the cladding 72is a buffer material 74 that provides strength and protection to theoptical fiber 32. In all embodiments herein, the buffer material isremoved from the optical fiber 32 throughout the length of the chirpedbroadband cladding mode coupler 34 and narrow-band core mode coupler 35.The buffer material 74 remains present prior to the chirped broadbandcladding mode coupler 34 and after the narrow-band core mode coupler 35to absorb the acoustic energy created by the first 38 and second 46acoustic wave amplifiers thereby limiting the length of the first 43 andsecond 44 acousto-optic interaction regions respectively. Additionally,the buffer material following the narrow-band core mode coupler 35rapidly attenuates the unselected cladding mode lightwaves.

The optical fiber 32 can support the propagation of multiple claddingmodes and a single core mode over a nominal wavelength range. In anembodiment of the present invention, the optical fiber 32 is tensionedsufficiently to keep the fiber 32 straight. Keeping the optical fiber 32straight reduces the loss of cladding mode lightwaves. Although multipleelectrical sinusoidal frequencies may be applied to the second acousticwave generator 37 to select multiple bandwidths, the difference betweenthe electrical sinusoidal frequencies should normally not be less than50 KHz. Electrical sinusoidal frequencies applied to the second acousticwave generator 37 and separated by less than 50 KHz cause amplitudemodulation of the optical signal passing through the bandpass filter 30.A more detailed explanation of the amplitude modulation phenomena causedby multiple electrical sinusoidal frequencies is given in U.S. Pat. No.6,233,379 by Kim.

The core 70 of the optical fiber 32 is substantiallycircularly-symmetric to ensure the refractive index of the core mode isessentially insensitive to the state of optical polarization. In atypical non-birefringent single-mode fiber (as in 32 herein), theeffective index difference between orthogonal polarization states istypically smaller than 10⁻⁵.

The chirped broadband cladding mode coupler 34 is composed of a firstacoustic-wave generator 36, a first acoustic wave amplifier 38, and afirst acousto-optic interaction region 43 of optical fiber 32. To couplelightwaves from a core mode to a cladding mode, the phase matchingcondition must be satisfied. The propagation constants of the opticalmodes can be changed by varying the diameter of the optical fiber 32. Inthe first acousto-optic interaction region 43 of optical fiber 32, theouter diameter of the cladding 72 as well as the outer diameter of thecore 70 is altered. As illustrated in FIG. 1, the first acousto-opticinteraction region 43 of the optical fiber 32 has an outer diameter thatchanges along its longitudinal length in a continuous, typically linear,fashion. Within this first acousto-optic interaction region 43, both thephase matching condition and the coupling strength are varied along itslongitudinal axis, and the phase matching condition for differentoptical wavelengths are satisfied at different positions along thelongitudinal axis. Coupling between the lower order core mode and ahigher order cladding mode then can take place over a wide opticalwavelength range. The size of the optical bandwidth can be controlled bythe degree of change in the diameter of the optical fiber 32 within thefirst acousto-optic interaction region 43. In addition, the couplingefficiency over a broad optical bandwidth can be controlled by extendingthe length of the fiber 32 at certain fiber diameters. By extending thelength of the fiber 32 at certain fiber diameters, the interactionlength is increased allowing for stronger coupling to occur. It ispreferable that most if not all of the optical lightwaves be removedfrom the core 70 of the optical fiber 32 to prevent optical interferencefrom occurring when the cladding modes are coupled back into the core bythe narrow-band core mode coupler 35.

The function of the chirped broadband cladding mode coupler 34 is tosignificantly couple the core-mode lightwaves of the optical fiber 32entering the bandpass filter 30 into cladding mode lightwaves. Althoughthe cladding 72 of the fiber 32 may support multiple modes, thelightwaves are coupled to a specific cladding mode, preferably of alower order mode (i.e. LP(1,1), (1,2), or (1.3)).

The entire broadband signal, having been coupled into a cladding mode inthe chirped broadband cladding mode coupler 34, now propagates parallelto the optical axis of the fiber 32 within the cladding 72 into anarrow-band core mode coupler 35 and second acousto-optic interactionregion 44 where selected bands of optical wavelengths are coupled fromthe fiber 32 cladding 72 back to the core 70. A typical bandwidth foreach of the selected bands of optical wavelengths being coupled backinto the core 70 is 0.4 2.0 nm. This coupling is tuned by varying theelectrical sinusoidal frequency of the second frequency source 54, whichis electrically coupled to a second acoustic-wave generator 37, which inturn couples acoustic energy through the second acoustic wave amplifier46 to the fiber 32. The second acousto-optic interaction region 44extends from the acoustic absorber 48 to the tip of the second acousticwave amplifier 41. The length of the fiber 32 in the secondacousto-optic interaction region 44 is typically less than 1 meter, andpreferably less than 20 cm. The uniformity of the fiber diameter andindex of refraction within the second acousto-optic interaction region44 maximizes coupling efficiency and minimizes spectral sidebands in thetransmission spectrum of the filter. Other issues regarding the lengthof this second acousto-optic interaction region 44 are discussed in U.S.Pat. No. 6,233,379 by Kim, previously incorporated as a reference.

To limit the acousto-optic interaction between the chirped broadbandcladding mode coupler 34 and the narrow-band core mode coupler 35, anacoustic absorber 48 is interposed surrounding the fiber 32 between thefirst acousto-optic interaction region 43 and the second acousto-opticinteraction region 44. The acoustic absorber 48, which can beconstructed from a variety of materials, is coupled to and surrounds thefiber 32 and has an index of refraction lower than the cladding 72 toprevent the cladding modes from being absorbed by the acoustic absorber48. The lower index of refraction will permit the lightwaves to continuewithin the cladding 72 without being disturbed by the acoustic absorber48. The acoustic absorber 48 significantly dampens any acoustic wavesand minimizes reflections of the acoustic waves. Any reflections ofacoustic waves would cause an intensity modulation of the optical signalpassing through the bandpass filter 30 by generating frequency sidebandsin the optical signal.

An alternative embodiment of an acoustic absorber 48 consists of a lowrefractive index thin layer 47 surrounding the fiber 32 with a highrefractive index acoustic absorber 49 coupled to the low refractiveindex thin layer 47 as shown in FIG. 5.

Referring to FIG. 6, the first acoustic wave amplifier 38 shown is anexemplar for both the first 38 and second 46 acoustic wave amplifiers.The design options discussed for the first acoustic wave amplifier 38apply to both the first 38 and second 46 acoustic wave amplifiers. Thefirst acoustic wave amplifier 38 may have a variety of differentgeometric configurations but is preferably elongated along the fiberlongitudinal axis. Typical longitudinal lengths of the first acousticwave amplifier are 5 15 millimeters. In various embodiments, the firstacoustic wave amplifier 38 is tapered linearly from the base 56 to thetip 40 and may be conical as illustrated in FIG. 6. The, typicallyconical, shape of the first acoustic wave amplifier 38 providesmagnification of the acoustic wave amplitude at the tip 40. Generally,the first acoustic wave amplifier 38 has a longitudinal axis that isparallel to the fiber 32, however the first acoustic wave amplifier 38may be coupled to the fiber 32 at any angle or even from the side suchthat the axis of the first acoustic wave amplifier 38 is nearlyorthogonal to the axis of the fiber 32. The first acoustic waveamplifier 38 may be made from a glass capillary, such as fused silica, acylindrical rod with a central hole, or the like. In another fashioning,a glass capillary is machined to form a cone and the flat bottom of thecone is bonded to an acoustic wave generator. To preserve the phase ofthe acoustic waves, the exterior surface of the first acoustic waveamplifier 38 is generally smooth. In other embodiments, the firstacoustic wave amplifier 38 is horn shaped with a diameter that decreasesexponentially from the base 56 to the tip 40.

In an embodiment of the bandpass filter 30, the first acoustic waveamplifier 38 has an interior containing a fiber receiving channel 58.The fiber receiving channel 58 can be a capillary channel with adiameter slightly greater than the outer diameter of the fiber 32 used.

In typical embodiments, a bonding agent 50 is positioned between thefiber 32 and the fiber receiving channel 58 at an interface in proximityto the tip of the first acoustic wave amplifier 40. Suitable bondingagents 50 include, but are not limited to, epoxy, glass solder, metalsolder, and the like. The bonding agent 50 is sufficiently rigid forefficient coupling of the acoustic waves from the first acoustic waveamplifier 38 to the fiber 32 and the bonding agent 50 is sufficientlyrigid to minimize back reflections of the acoustic waves from the fiber32 to the first acoustic wave amplifier 38.

The dimensions of the fiber receiving channel 58 and the outer diameterof the fiber 32 are sufficiently matched to place the two in closeproximity so as to minimize the amount of bonding agent 50 needed. Therelative sizes of the fiber 32 and the fiber receiving channel 58 needonly be substantially the same at the interface. The difference in thediameter of fiber 32 and fiber receiving channel 58 is in the range of1-10 microns.

Although various preferred embodiments of the present invention havebeen described herein in detail to provide for complete and cleardisclosure, it will be appreciated by those skilled in the art, thatvariations may be made thereto without departing from the spirit of theinvention or the scope of the appended claims.

What is claimed is:
 1. A chirped fiber acousto-optic bandpass filter,comprising: a non-birefringent single-mode optical fiber, with alongitudinal axis, core, concentric cladding over the core, and proximaland distal ends; a broadband cladding mode coupler further comprising afirst longitudinal length of the optical fiber closer to the proximalend of the optical fiber, wherein said first longitudinal length has achirped diameter, and at least one first acoustic-wave generator coupledto the optical fiber within the first longitudinal length; a narrow-bandcore mode coupler further comprising a second longitudinal length of theoptical fiber closer to the distal end of the optical fiber and at leastone second acoustic-wave generator coupled to the optical fiber withinthe second longitudinal length; and an acoustic absorber deployed andcoupled to the optical fiber between said broadband cladding modecoupler and said narrow-band core mode coupler.
 2. The filter of claim1, wherein the diameter of said second longitudinal length of theoptical fiber is tapered.
 3. The filter of claim 1, wherein said firstlongitudinal length of the optical fiber, in addition to having achirped diameter, also has a tapered diameter.
 4. The filter of claim 1,wherein said optical fiber is tensioned along its longitudinal axis. 5.The filter of claim 1, wherein the chirped diameter of the optical fiberwithin the first longitudinal length is held constant at someintermediate chirped diameter for some length of the portion of theoptical fiber having a chirped diameter.
 6. The filter of claim 1,wherein said acoustic absorber is further comprised of a low refractiveindex thin layer of material surrounding the optical fiber and a highrefractive index acoustic absorber coupled to the low refractive indexthin layer of material.
 7. The filter of claim 1, wherein at least oneof said first acoustic-wave generators is coupled to the optical fiberwith a bonding agent.
 8. The filter of claim 1, wherein at least one ofsaid second acoustic-wave generators is coupled to the optical fiberwith a bonding agent.
 9. The filter of claim 1, wherein at least one ofsaid first acoustic-wave generators has an exterior diameter that iselongated along the optical fiber longitudinal axis.
 10. The filter ofclaim 1, wherein at least one of said second acoustic-wave generatorshas an exterior diameter that is elongated along the optical fiberlongitudinal axis.
 11. The filter of claim 1, wherein at least one ofsaid first acoustic-wave generators is electrically connected to atleast one frequency source.
 12. The filter of claim 1, wherein at leastone of said second acoustic-wave generators is electrically connected toat least one frequency source.
 13. The device of claim 1, wherein thediameter of said second longitudinal length of the optical fiber isuniformly tapered.
 14. The filter of claim 1, wherein said firstlongitudinal length of the optical fiber, in addition to having achirped diameter, also has a uniformly tapered diameter.